1. Field of the Invention
The invention relates to H-bridges, and more particularly to Electro-Magnetic-Interference (EMI) of H-bridges.
2. Description of the Related Art
A switching amplifier, which is referred to as a class-D amplifier, is frequently used as an audio amplifier due to high power conversion efficiency. The power conversion efficiency of a switching amplifier can achieve 95% or higher while that of a class-AB amplifier is usually less than 80%. Thus, a switching amplifier has power conversion efficiency higher than that of the class-AB amplifier, and a system using a switching amplifier has a longer battery playback time than that of a system using a class-AB amplifier.
The major drawback of a switching amplifier is related to high electro-magnetic interference (EMI). The EMI problem can be solved by using an L-C (inductor-capacitor) filter. The side effect is that the inductor is bulky, non-linear, and consumes power (inductor has inherent parasitic resistance). The rapid switching mechanism of a switching amplifier induces an electro-magnetic interference (EMI) problem that can interfere and cause errors to the electronic circuits near the switching amplifier.
A switching amplifier tied with a load forms a circuit with a shape of the character H and is referred to as an H bridge. Referring to FIG. 1, a circuit diagram of a conventional H-bridge is shown. The H bridge has a first output node OUTP and a second output node OUTM, and a load is coupled between the output nodes OUTP and OUTM of the H bridge. The H bridge comprises four switches M1, M2, M3, and M4. The first switch M1 is coupled between a voltage source VDD and the first output node OUTP, the second switch M2 is coupled between the first output node OUTP and ground GND, the third switch M3 is coupled between the voltage source VDD and the second output node OUTM, and the fourth switch M4 is coupled between the second output node OUTM and ground GND.
The switches M1, M2, M3, and M4 respectively have body diodes DM1, DM2, DM3, and DM4. Four control signals GD1, GD2, GD3, and GD4 are respectively coupled to the gates of the transistors M1, M2, M3, and M4 to turn off or turn on the transistors M1, M2, M3, and M4. The control signals GD1, GD2, GD3, and GD4 are generated according to pulse-width modulation (PWM). Among different modulation schemes of PWM, BD modulation is widely used due to lower EMI and high efficiency. The advantage of this invention over conventional BD modulation will be explained in detail. In a conventional Class-BD PWM, the voltages on the output nodes OUTP and OUTM, the voltage across the load (OUTP−OUTM), and the common mode voltage ½(OUTP+OUTM) are determined by the On-off states of the switches GD1, GD2, GD3, and GD4 and are summarized in the following table:
TABLE 1VoltageCommonVoltage atVoltage atacrossmodeM1M3M2M4OUTPOUTMloadvoltageOnOnOffOffVDDVDD0VDDOnOffOffOnVDDGNDVDD½VDDOffOnOnOffGNDVDD−VDD½VDDOffOffOnOnGNDGND00
The common mode voltage also has three levels: VDD, ½VDD, and 0. The common mode voltage switches with the PWM clock, and the fast switching wave produces significant EMI radiation in the 30-1000 MHz range. This EMI can be suppressed by the LC filters coupled between the output nodes (OUTP and OUTM) and the GND. The added filters increase the cost and size of the switching system. Hence it is desirable to find solutions to reduce common mode switching noise. In addition, rapid fluctuation of voltages on the output nodes OUTP and OUTM induces large EMI effects which further degrades performance of the switching system. Thus, a new switching system avoiding rapid fluctuation of voltages on the output nodes OUTP and OUTM is therefore required.